Current TWC catalysts are used for mobile emission control from Otto engines. The technology is well developed with emission reduction capabilities of >99% for CO, HC (hydrocarbons) and NOx (nitrogen oxides) after heat up to operating temperatures of greater than 250° C. Typical TWC catalyst configurations consist of single brick or multi-brick systems in the exhaust line of the vehicle. If more than one catalyst is used, the catalysts can be located in a single converter, butted together, or separated by a defined space as in separate converters. A common design for large engines is to have one converter in a hot close coupled (CC) position (close to the manifold) with the second converter in the cooler underbody (UB) location. Since nearly all mobile emission control systems are passive in nature, time to heat up to the catalyst operating temperature is critical as disclosed in EP1900416, which is relied on and herein incorporated by reference in its entirety.
Thus, CC catalyst designs often consist of features that favor rapid heat up such as light, small size substrates (low thermal inertia), high cell density (improved mass & heat transfer) and high platinum group metal (PGM; e.g., platinum, palladium, rhodium, rhenium, ruthenium and iridium) loading. On the other hand the UB catalyst can be of larger volume and lower cell density (lower pressure drop) and more often contains lower PGM loading. For smaller vehicles that operate at high RPM only one converter is typically used, often located in the CC position. A disadvantage of locating catalysts close to the manifold is increased thermal degradation, and more rapid loss of activity, especially under high load/high speed conditions which results in loss of support surface area or pore volume and rapid sintering of the PGM.
Modern TWC catalysts use a variety of strategies to limit or slow thermal degradation such as high surface area stable alumina supports for the PGMs, the addition of promoters and stabilizers and advanced oxygen storage components (OSCs) that both improve performance and degrade at a slower rate (see e.g. U.S. Pat. No. 5,672,557, which is relied on and herein incorporated by reference in its entirety).
In the art, certain design strategies have been used to balance performance with associated costs. These strategies include selection of PGM type and distribution, substrate volume, cell density, WC layering, and composition of the various WC layers.
An important design feature for TWC technologies consists of appropriate separation and configuration of both the PGM and washcoat (WC) components either in separate WC layers and/or in separate bricks if multi-brick systems are used. Most modern TWC catalysts can have one to more WC layers, the most common being 2-layer systems. See e.g., EP1541220, U.S. Pat. No. 5,981,427, WO09012348, WO08097702, WO9535152, U.S. Pat. No. 7,022,646, U.S. Pat. No. 5,593,647, which are relied on and herein incorporated by reference in their entirety.
For the PGMs, the most common strategy is to locate the Rh and optionally the Pt component in the top or 2nd WC layer with Pd preferably located in the bottom or 1st WC layer (see e.g., U.S. Pat. No. 5,593,647). Separation of both the WC components and PGMs can also be achieved for single bricks by zoning whereby the front or rear zone or section of a WC layer can consist of different support components or different PGM components or more commonly different concentrations of a given PGM such as Pd. One advantage for separation of the PGMs in layers or zones is that more optimum supports and promoters for each PGM can be used so as to maximize overall performance.
Prior to the present invention, researchers have been drawn to certain WC composition configurations that are taught as representing the preferred configuration for best performance. Thus, for two-layer UB catalysts Rh is invariably located in the top (2nd) layer with optionally Pt also present while Pd is located in the 1st or bottom layer (see e.g., U.S. Pat. No. 5,593,647). Further, both the top (2nd) and bottom (1st) layers ideally contain a high surface area refraction oxide support such as a gamma or gamma/theta/delta alumina with further addition of promoters, stabilizers and a suitable oxygen storage component (OSC). This WC design is described in detail by Sung et al. (U.S. Pat. No. 6,087,298) and Hu et al. (U.S. Pat. No. 6,497,851) hereby included for reference purposes. Both Sung et al. and Hu et al. also describe preferred WC compositions and configurations for the CC catalysts or zones at the inlets to the exhaust gas flow. Thus, for the inlet CC or inlet (front) zone the WC design is preferably free of an OSC and consists of a high surface area refractory oxide support such as a gamma or theta/delta alumina with appropriate stabilizers and additives. On the other hand, it is preferred that the rear catalyst, zone or UB catalyst, have an OSC present in the bottom and top layers. These and other features are described for example by Hu et al. and references quoted therein.
Within the TWC catalyst field new technologies and WC configurations and systems are required to meet the ever more stringent emission standards and the need to slow catalyst deactivation and achieve ever increasing performance at low PGM loadings.